With recent developments in magnetic recording, the magnetic recording media which can reproduce images and sounds of higher quality have been strongly demanded. In order to meet these demands, a reduction in the particle size of the ferromagnetic particles and an increase in the density of the magnetic recording media have hitherto been promoted. Further, since the magnetic recording media are consumed in large amounts, they have been required to be produced at a lower cost. One technique to meet these demands is to provide a plurality of the magnetic layers. This technique is advantageous for increasing the density in that shorter-wavelength recording characteristics can be imparted to an upper layer and longer-wavelength recording characteristics can be imparted to a lower layer, thereby using suitable ferromagnetic particles in the respective layers. In the use for shorter-wavelength recording alone, the upper magnetic layer must be decreased in thickness and a nonmagnetic particle must be used in the lower layer, thereby diminishing self-demagnetization, which advantageously results in a higher-density recording medium. At the same time, the recording medium having plural magnetic layers has a feature that the medium can be produced at a low cost because suitable materials can be used depending upon the respective layers. Recently, methods for improving the surface properties of the magnetic layers, or ferromagnetic particles high having output and low noise have been required to improve electromagnetic characteristics.
On the other hand, such electromagnetic characteristics of the magnetic recording media should be evaluated on the condition that the magnetic recording media have good running ability above a certain level. In order to secure good running ability, therefore, the surfaces of the magnetic layers and the backing layers of the magnetic recording media have been required to be low in the coefficient of friction. For electrification which is the main cause of drop out or an output defect, it has been necessary that the magnetic recording media are hard to be charged.
That is, the requirements for the present magnetic recording media are as follows:
(1) the magnetic recording media have excellent electromagnetic characteristics; PA1 (2) the magnetic recording media are hard to be charged; PA1 (3) the magnetic recording media have excellent running durability; and PA1 (4) the magnetic recording media have excellent productivity. PA1 (1) polyalkylene glycol; PA1 (2) sulfonic acid metal salt derivative; and PA1 (3) at lease one of aromatic amine and ammonium salt thereof. PA1 (1) polyalkylene glycol; PA1 (2) sulfonic acid metal salt derivative; and PA1 (3) at lease one of aromatic amine and ammonium salt thereof; and the ferromagnetic particle is a ferromagnetic alloy particle. PA1 (1) polyalkylene glycol; PA1 (2) sulfonic acid metal salt derivative; and PA1 (3) aromatic amine and/or ammonium salt thereof. PA1 (a) a method in which a complex organic acid salt (mainly an oxalic acid salt) is reduced with a reducing gas such as hydrogen; PA1 (b) a method in which iron oxide is reduced with a reducing gas such as hydrogen to obtain Fe or Fe--Co particles; PA1 (c) a method in which a metal carbonyl compound is thermally decomposed; PA1 (d) a method in which a reducing agent such as sodium borohydride, hypophosphate or hydrazine is added to an aqueous solution of a ferromagnetic metal to conduct reduction; PA1 (e) a method in which a ferromagnetic metal particle is electrolytically precipitated by the use of a mercury cathode, followed by separation from mercury; and PA1 (f) a method in which a metal is vaporized in a low-pressure inert gas to obtain a fine particle.
First, a surface of the magnetic layer is required to be made as smooth as possible (for example, JP-A-57-130234 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-61-168124). For this purpose, a base surface on which the magnetic layer is formed is required to be very smooth. In particular, when the magnetic layers are formed in a multiple-layer structure, a surface of a nonmagnetic support on which the magnetic layers are formed influences the surface properties of the lower magnetic layer, which further influences the surface properties of the upper magnetic layer. Accordingly, the surface properties of the surface of the nonmagnetic support on which the magnetic layers are formed is important, and the surface is required to be smooth. Further, in order to obtain excellent electromagnetic characteristics, magnetic substances are required to be high in output and low in noise.
Next, electrification of a magnetic recording tape induced by sliding the tape on a loading guide for the tape and a cylinder in a VTR or on cassette members in a cassette causes easy adhesion of a powder falling from the tape itself, a powder produced from the cassette members or a surface of the tape by wear due to the contact of the tape with the cassette members and dust from the outside to the tape, which results in partial space loss on recording and reproduction of a magnetic head, leading to a drop-out failure. For the purpose of preventing this, a reduction in the surface electrical resistance of the surfaces of the magnetic layers (for example, JP-A-59-16140 and JP-A-59-63029) and a reduction in the surface electrical resistance of the back coating layers formed on the surfaces opposite to the surfaces of the nonmagnetic supports on which the magnetic layers are formed (for example, JP-A-57-150132 and JP-A-59-3722) have been known. However, the formation of the back coating layers requires the step of forming the back coating layers and the development of back coating solutions, resulting in a high production cost, and brings about powder dropping and separation of the back coating layers on repeated use, resulting in deteriorated durability.